Iron powder for forming sintered articles of improved strength



Dec. 24, 1968 HATHER ETAL 3,418,105

IRON POWDER FOR FORMING SINTERED ARTICLES OF IMPROVED STRENGTH Filed April 19. 1966 TRANSVERSE RUPTURE STRENGTH OF SINTERED IRON TEST BARS AT A 6.5 gm/c0 SINTERED DENSITY ASA FUNCTION 0 rm; PARTICLE SIZE.

TRANSVERSE RUPTURE STRENGTH cesx.)

50 6O 7 80 9O 25O MESH (70) TRANSVERSE RUPTURE STRENGTH (i? S. I.)

40 5O 60 P0 IN VENT 0R "3 MESH 7 Rarrq R. HGT 6176f United States Patent 3,418,105 IRON POWDER FOR FORMING SINTERED ARTICLES OF IMPROVED STRENGTH Harry R. Hatcher, Crown Point, Ind., and William Holtzman, Wilkes-Barre, Pa., assignors, by mesne assignments, to SCM Corporation, New York, N.Y., a corporation of New York Filed Apr. 19, 1966, Ser. No. 543,570 Claims. (Cl. 75.5)

The present invention relates to powder metallurgy and, more particularly, to a compactible iron metal powder adapted to form sintered articles of improved strength, especially to rupture.

Within recent years, the practice of fashioning machine parts or the like from metallic iron powder has undergone significant growth. Articles produced by metal metallurgy commonly replace articles of cast iron or steel formed by the more conventional casting or machining operations. For example, compactible iron powder is used for the fabrication of a wide variety of structural parts such as sleeves, bushings, gears, etc. For this purpose, the compactible iron powder must possess a wide range of satisfactory attributes including apparent density, adequate dry flow, acceptable tensile strength, modulus of rupture when sintered under a variety of atmospheric conditions following preliminary compaction, and still other acceptable physical characteristics.

In general, the iron powder is compressed prior to sintering into substantially the exact shape and size desired in a finished product, in this way dispensing with expensive machining operations. After compression, which may be at a pressure up to 100,000 pounds per square inch, the resulting shape is heated at an exemplary temperature of about 2,000 P. to about 2,100" F., preferably under non-oxidizing conditions, for sufiicient time to produce a sintered article.

In general, to produce a compactible iron powder, a suitable iron-carbon alloy, either alone or in mechanical mixture with free iron, is comminuted and then decarburized as by reduction. For instance, a mixture of a powdered iron-carbon alloy and a powdered oxide of iron is treated under such conditions that the carbon of the alloy combines with the oxygen of the oxide to reduce the latter to iron while the iron-carbon alloy is simultaneously decarburized.

Throughout the continuous growth of the art of powder metallurgy, attempts have been made to produce superior, compactible iron powders which produce, in turn, superior, sintered articles. In Patent 2,402,120 to Boegehold et al., for example, an iron powder mixture is disclosed as composed of solid, irregularly shaped iron powder particles and porous, spongy, irregularly shaped iron powder particles. By utilizing suitable mixtures of these two classes of powders, the size and shape of the pores in the ultimately sintered article are said to be controlled so as to produce in the sintered article a controlled porosity of especial advantage where an impregnant is to fill such pores.

It has now been discovered that the degree of grinding of the raw materials defining the reducible :feed stock plays a significant role in the strength of the final, sintered article. In particular, an improved feed stock mixture containing an iron-carbon alloy and iron oxide can be obtained for producing compactible iron metal pow- 3,418,105 Patented Dec. 24, 1968 der by (a) using a particular iron carbide-containing stock which is a hypereutectic composition consisting essentially of primary platelets of iron carbide in a matrix of iron carbide-gamma iron eutectic and (b) insuring that at least about 72 percent 'by weight of such feed stock mixture has a particle size passing 250 mesh. Preferably, at least about 52 percent of such feed stock passes 325 mesh, and no more than about one percent of the feed stock in coarser than mesh. All references here and in the claims to a mesh size are with respect to the Tyler scale.

When these considerations are observed in admixing a powdered ferrug-inons charge prior to its reduction, the resulting compactible metal iron powder produces sintered articles of improved strength, particularly to transverse rupture. The accompanying FIGURES 1 and 2, hereinafter more fully described, demonstrate the marked improvement in transverse rupture strength in sintered articles as the quantity of the progenitor iron carbideiron oxide charge mixture (from which the iron powder was prepared) increases with respect to the 250 mesh and 325 mesh sizes. In both instances, there is a pronounced inflection in the plotted curves after which the rupture strength rises rapidly. In the case of the 250 mesh (FIG- URE 1), the inflection point, and therefore the predominantly critical particle size, is about 72 percent through such mesh; and in the case of 325 mesh, the inflection point is about 52 percent through that mesh.

Referring more particularly to the iron carbide composition, this hypereutectic consists essentially, as indicated, of coarse grain primary platelets of iron carbide in a matrix of iron carbide-gamma iron eutectic which is preferably :free of graphite and contains about 4.1 percent to about 4.6 percent carbon. The intrusions of platelets of iron carbide into the matrix (which can be observed under a microscope) contribute materially to the grindability or frangibility of the eutectic. This is quite important to easily powdering the composition and realiz ing the specific particle sizes previously mentioned. The amount of carbon present can also be important. If less than about 4.1 percent carbon is available, an insufiicient number of the platelets is formed which deleteriously affects the desired frangibility. On the other hand, if more than about 4.6 percent carbon is present, graphite is apt to form, as during quenching of the melt which produces the hypereutectic composition. A small quantity of graphite can be tolerated, but for best results none should be present. Graphite tends to nucleate and form still more graphite.

A convenient source of the present hypereutectic composition is what is known in the art as white iron. This type of iron can be produced by rapidly quenching a melt of a high carbon-iron alloy. A brittle structure results that is low in austenite and high in cementite, pearlite, and martensite. The brittleness contributes to the desired frangibility of composition.

In particular, sorel metal admirably provides a source of the present hypereutectic composition. Sorel metal can be formed during the electric arc smelting of iron oxide in the presence of carbon, as distinguished for example from cupola-made white cast iron.

An iron oxide-containing ferruginous material is added to and thoroughly mixed with the eutectic composition to reduce the iron carbide and simultaneously provide additional free metallic iron from the oxide. Any iron oxide can be used, such as naturally occurring ores like magnetite, hematite, and limonite. Similarly, partially reduced iron oxides having about two percent to about 30 percent hydrogen loss (as determined by conventional M.P.I.F. test) can also be used. A quite inexpensive and readily available source of an iron oxide is mill scale which comprises mixed iron oxides containing about 75 percent iron. The relative amounts of the iron carbide and iron oxide in the free iron mix are not at all critical. Ideally, at least suificient oxide is used stoichiometrically to remove all of the carbon, although as previously indicated, complete removal of the carbon is not essential as when the presence of a little (less than about 1.5 pere c n betq erat When h pr d hydro n o dissociated ammonia atmosphere is used for reduction, it can be relied on for reduction of some of the feed mixture apart from the iron carbide-iron oxide reaction. In practice, from about 60 to 85 parts by weight of a sorel meta-l have been mixed with from about to about 40 parts by weight of mill scale. A ratio of about 80 parts of sorel metal to about parts of mill scale is preferred.

The iron carbide and iron oxide feed stocks are ground, preferably separately for grinding control, by conventional means such as ball milling to a particle size within the range noted, after which the ground particles are thoroughly and intimately mixed as previously indicated and heated under nonoxidizing conditions to a temperature at which interaction of the carbide with the iron oxide yields compactible, metallic iron powder.

Reduction of the feed stock mixture can be done convention aly in a single stage or a plurality of stages. Two reduction stages are preferred for efficiency and economy of operation. A single reduction stage or the first reduction stage, in which the bulk of the reduction is .done, can be operated at a temperature of about 16002000 F. with effectiveness and preferably is operated at 1700- 1750 F. for efficiency and economy. The second stage, in which only a small amount of finishing reduction is done, can be operated at an even higher temperature if desired without danger of oversintering or liquefying the particles, and it can be effective at a temperature as low as about 1350 F., although the most eflicient temperature is l7001750 F.

The atmosphere in the reduction should be neutral to the material in process (e.-g., preponderantly nitrogen, argon, helium, or the like), or preferably reducing to give more etficiency and flexibility to the operation (pure hydrogen, hydrogen containing small proportions of nitrogen, and dissociated ammonia). The reduction best is done in the substantial absence of molecular oxygen or materials capable of forming molecular oxygen under normal operating conditions. A typical gas feed to supply a good atmosphere for the operation is 75 volume percent hydrogen, volume percent nitrogen having a dewpoint of 39 F.

The preferred apparatus for reduction is a thin layer continuous belt furnace, although the reduction can be performed batchwise or in trays or saggers passed through a tunnel kiln. A slight positive pressure generally is maintained in reduction to prevent air from coming in the apparatus. If the apparatus can be made tight, a high vacuum can be used, the rarefield atmosphere thus provided being suitable in itself as a neutral atmosphere when venting of gaseous byproducts is practiced.

The following examples only illustrate the invention and should not be construed in any manner to limit the claims. All percentages indicated are by weight. In the case of screen analysis, the percentage listed represents the amount by weight passing through the indicated screen size.

Example 1 The following is a working example illustrating the technique followed in exemplary operation. A sorel metal and mill scale were used having the chemical and screen analyses as given by Table A.

TABLE A Chemical Analysis Sorel metal Mill Scale Carbon wt. percent 4.2-4.4 0.30 max. Sulfur wt. percent..... 0.025 max 0.05 max. Phosphorus wt. percent 0.03 max....

Copper wt. percent Silicon wt. percent Manganese wt. percent Titanium wt. percent Hydrogen Loss, percent Acid Insolubles wt. percent 2 0.08 max 0.10 max. 0.20 max 0.15 max Moisture Screen Analysis:

On 3 mesh wt percent 5.0 max On 4 mesh, 10.0 max. On 4 mesh wt percent 10.0 max On 10 mesh, 5-15. On 8 mesh wt. percent 30.0 max".-- On 20 mesh, 15-25. On 14 mesh wt. percent 50.0 max On 35 mmh, 20-35. On 28 mesh wt. percent 5-30 On 48 mesh, 10-20.

' Through 28 mesh wt. peroent "5-25. 011 mesh, 3-10.

Through 60 mesh,

petermined in accordance with M.P.I.F. Std. 2-64, issued 1948, l fe t i i iiined in accordance with M.P.I.F. Std. 6-64, issued 1954, rcvised1964.

Each component was separately dried and then separately ground in a ball mill. In each case, grinding was continued until about 72 percent to about percent passed 325 mesh, and about 0.2 percent to about 1.0 percent was retained on mesh. The sorel metal and. mill scale were next thoroughly and intimately mixed together in a weight ratio of 4:1, respectively, and then briquetted by a conventional briquetting machine at a pressure of about 2400 p.s.i. to about 2600 p.s.i.

The briquets were reduced in a thin layer continuous belt furnace at a temperature of 1750 F. in an atmosphere of dissociated ammonia. The bed of the furnace was a moving belt travelling at 5% inches per minute to yield a reduction time at temperature stated of about 10 minutes. The approximate feed rate in pounds per hour was 1325. The bed depth of briquets being reduced was about 1% inches. The intermediate product emerged continuously from the furnace at an approximate rate of 1165 pounds per hour, having a hydrogen loss of 1.50 max. and carbon content of 0.30 max. This intermediate product was ground in an attrition mill in an atmosphere of inert gas to particles passing 60 mesh and having a density between about 2.2 and 2.4 grams per cc.

The intermediate product then was sent through a second thin layer furnace maintained at 1750 F. The belt speed in the second furnace was about six inches per minute and the bed depth was about 1% inches to yield a holding time at temperature stated of about 2025 minutes. Particle feed was continuous at the rate of about 1200 pounds per hour. The product output came off the furnace continuously at about 1160 pounds per hour. The product was ground in inert atmosphere so that it passed an 80 mesh screen. The product had carbon content of 0.08 max., a hydrogen loss of 0.50 percent max., an apparent density of 2.25-2.40 grams per cc., and 35 percent max. of the particles were finer than 325 mesh.

This resultant iron powder product could then be used to form shaped articles by standard pressing and sintering operations.

Example 2 Two grades for each of the iron carbide and the mill scale components were used as raw materials in preparing test samples, the grades of like components dilfering essentially from each other in particle size only. Table B provides the chemical and screen analyses for the relatively coarse and fine iron carbides (S and 5:, respectively), and for the relatively coarse and fine mill scale (M and M respectively). Four test samples, numbered Mix No. 1 through Mix No. 4, were prepared from the components of Table B. Each mix had a ratio of 80 parts of iron carbide to 20 parts of mill scale, but the various coarse and fine grades were intermixed as shown by Table C. The particle size of each resulting mix was also determined, as shown by Table C, to the extent of determining the amount of each component of each mix passing through 250 mesh and also, through 325 mesh.

TABLE B.-RAW MATERIALS S Sr Me Mr Chemical Analysis Iron Iron Mill Mill Carbide Carbide Scale Scale Carbon 3. 97 3. 81 40 3. 71 Hydrogen Loss. 2. 93 4.06 25. 64 24. 96 Acid Insolubles 72 38 Manganese-.. 18 07 22 51 Sieve Analysis, Tyle +100 mesh- 0.1 100 +150 mesh-.- 10. 2. 5 150 +200 mesh 17.8 8.0 200 +250 10. 5 6.0 250 +325 mesh 22. 2 20. 4 325 mesh 38. 9 63. 1 Percent 250 61.1 83. 5

TABLE C.-MIXED REACTANTS Mix #1 Mix #2 Mix #3 Mix #4 Iron Carbide, 80% So 80% Se 80% Sr 80% S; Mill Scale Mo 20% Mr 20% Me 20% Mi Percent 325:

Iron Carbide 31. 1 31. 50. 5 50. 5 Mill Scale"-.. 7. 3 18. 2 7. 3 18. 2 Percent 325 38. 4 49. 3 57. 8 68. 7 Percent 250:

Iron Carbide 48. 9 48. 9 68. 8 68. 8 Mill Scale. 10.6 20.0 10. 6 20.0 Percent 250 59. 5 68. 9 79. 4 88.8

Mix numbers 1 through 4 were separately processed and reduced at 1750 F. as described in Example 1. Each of the resulting iron powders was assigned a corresponding number. For example, Iron No. 1 was obtained from Mix No. 1. Table D indicates the chemical and screen analyses for these powders. Each of such iron powders was mixed with one percent of zinc stearate as a lubricant to facilitate compressing the powders at 27 tons per square inch into rupture test bars of uniform size. The test bars were sintered for about minutes at about 2050" F. in an atmosphere of dissociated ammonia. Table E shows the test results on the resulting bars.

TABLE D Iron Powder Produced Chemical Analysis Iron Iron Iron Iron No. 1 No.2 No. 3 No. 4

Carbon, percent 005 09 005 13 09 31 08 26 18 20 30 34 Manganese, percent. l6 10 19 17 Sieve Analysis, Tyler, percent:

+100 mesh 1. 9 0. 4 0.7 0.2 100 +150 mesh- 12. 5 10. 1 4. 4 2. 5 150 +200 mesh- 18. 8 16. 3 9.1 7. 7 200 +250 mesh. 10.9 9. 6 6. 7 5. 9 250 +325 mesh 22. 5 23. 1 22. 5 21. 8 325 mesh 33. 4 40. 5 56. 6 61. 9 Apparent Density, gins/cc. 2. 60 2. 57 2. 58 2. 38 Flow Rate, sec/50 gms 33.3 33.9 32.4

TABLE E Test Results Iron Iron Iron Iron No.1 N0. 2 No.3 No.4

Green Density, gms./cc 6. 45 6. 38 6. 36 6. 29 Green Strength, p.s.i 1, 498 1, 786 1, 910 2,123 Sintered Density, gms./cc- .45 6. 51 6. 37 6. 43 Sintered Strength, p.s.i 40, 463 43, 967 47, 003 50, 753 Dimensional Change, perce 12 0. 62 0. 14 0. 76 Sintered Strength corrected to 6.5

sintered density 42,100 43, 000 51, 600 53, 300

All data for the transverse rupture strength were corrected to a sintered density of 6.5 grams per cubic centimeter for comparison purposes. Such data was then plotted against percentage through 250 mesh and also against percentage through 325 mesh, as illustrated, respectively, by FIGURES 1 and 2. The curves of these figures indicate that there is a substantial increase in sintered strength, if the combined total percentage of the original feed stock passing 250 mesh exceeds about 72 percent. The same marked increase is shown if the feed stock is evaluated against the percentage passing 325 mesh where the critical point is about 52 percent.

Accordingly, the degree of grinding prior to reduction of the raw materials in an iron carbide-iron oxide feed stock mixture for producing compactible iron metal powder dictates to a critical and surprising extent the strength of a sintered product produced from such powder. This represents an important breakthrough toward the goal of realizing an improved compacting iron powder.

While the foregoing describes several embodiments of the present invention, it is understood that the invention may be practiced in still other forms within the scope of the following claims.

What is claimed is:

1. A feed stock for producing compactible iron metal powder adapted to form sintered articles of improved strength, said feed stock consisting essentially of a pow dered mixture of iron, iron carbide, iron oxide and minor amounts of components not affecting the properties thereof, the iron carbide being present in a hypereutectic composition consisting essentially of primary platelets of iron carbide in a matrix of iron carbide-gamma iron eutectic, and at least about 72 percent by weight of said powdered mixture having a particle size passing 250 mesh.

2. The feed stock of claim 1 wherein, additionally, at least about 52 percent of said powdered mixture is of particle size passing 325 mesh, and no more than about one percent is coarser than mesh.

3. The feed stock of claim 2 wherein said hypereutectic composition is a sorel metal formed as a product of the about 4.1 percent to about 4.6 percent by weight of carbon.

4. The feed stock of claim 3 wherein said hypereutectic composition is a Sorel metal formed as a product of the electric arc smelting of iron oxide-containing substance in the presence of carbon.

5. The feed stock of claim 1 wherein said iron oxide is mill scale.

6. In a process for producing iron metal powder by heating a pulverulent reaction mixture of iron, iron carbide, and iron oxide under nonoxidizing conditions and at a temperature at which interaction of said carbide with said oxide yields solid metallic iron, the improvement which comprises: using the iron carbide in the form of a hypereutectic composition consisting essentially of primary platelets of iron carbide in a matrix of iron carbidegamma iron eutectic; and forming the pulverulent reaction mixture from particles of which at least about 72 percent pass 250 mesh.

7. The process of claim 6 wherein, additionally, at least about 52 percent of said reaction mixture is of particle size passing 325 mesh, and no more than about one percent is coarser than 100 mesh.

8. The process of claim 7 wherein said hypereutectic composition is substantially free of graphite and contains about 4.1 percent to about 4.6 percent by weight of carbon.

9. The process of claim 8 wherein said hypereutectic composition is sorel metal formed as a product of the electric arc smelting of iron oxide-containing substance in the presence of carbon.

10. The process of claim 8 wherein said iron oxidecontaining ferruginous material is mill scale.

References Cited UNITED STATES PATENTS 3,073,695 1/1963 Silbereisen et al 75-211 3,194,658 7/1965 Storcheirn 75-211 3,214,262 10/1965 Von Bogdandy et al. 7521l 3,326,676 6/1967 Riibel et al. 75-201 3,368,890 2/1968 Schroeder et a1 75.5

L. DEWAYNE RUTLEDGE, Primary Examiner.

WAYLAND W. STALLARD, Assistant Examiner.

U.S. Cl. X.R. 75204 

1. A FEED STOCK FOR PRODUCING COMPACTIBLE IRON METAL POWDER ADAPTED TO FORM SINTERED ARTICLES OF IMPROVED STRENGTH, SAID FEED STOCK CONSISTING ESSENTIALLY OF A POWDERED MIXTURE OF IRON, IRON CARBIDE, IRON OXIDE AND MINOR AMOUNTS OF COMPONENTS NOT AFFECTING THE PROPERTIES THEREOF, THE IRON CARBIDE BEING PRESENT IN A HYPEREUTECTIC COMPOSITION CONSISTING ESSENTIALLY OF PRIMARY PLATELETS OF IRON CARBIDE IN A MATRIX OF IRON CARBIDE-GAMMA IRON EUTECTIC, AND AT LEAST ABOUT 72 PERCENT BY WEIGHT OF SAID POWDERED MIXTURE HAVING A PARTICLE SIZE PASSING 250 MESH. 